Re: [PATCH -mm v2 1/3] slub: never fail to shrink cache

[Sasha Levin <sasha.levin@oracle.com>]

Hi Vladimir,

On 01/28/2015 11:22 AM, Vladimir Davydov wrote:
> SLUB's version of __kmem_cache_shrink() not only removes empty slabs,
> but also tries to rearrange the partial lists to place slabs filled up
> most to the head to cope with fragmentation. To achieve that, it
> allocates a temporary array of lists used to sort slabs by the number of
> objects in use. If the allocation fails, the whole procedure is aborted.
> 
> This is unacceptable for the kernel memory accounting extension of the
> memory cgroup, where we want to make sure that kmem_cache_shrink()
> successfully discarded empty slabs. Although the allocation failure is
> utterly unlikely with the current page allocator implementation, which
> retries GFP_KERNEL allocations of order <= 2 infinitely, it is better
> not to rely on that.
> 
> This patch therefore makes __kmem_cache_shrink() allocate the array on
> stack instead of calling kmalloc, which may fail. The array size is
> chosen to be equal to 32, because most SLUB caches store not more than
> 32 objects per slab page. Slab pages with <= 32 free objects are sorted
> using the array by the number of objects in use and promoted to the head
> of the partial list, while slab pages with > 32 free objects are left in
> the end of the list without any ordering imposed on them.
> 
> Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>

It seems that this patch causes shrink to corrupt memory:

# echo 1 > /sys/kernel/slab/kmalloc-64/shrink
#
[   60.331433] =============================================================================
[   60.333052] BUG kmalloc-64 (Not tainted): Padding overwritten. 0xffff880051018f50-0xffff880051018fff
[   60.335714] -----------------------------------------------------------------------------
[   60.335714]
[   60.338530] Disabling lock debugging due to kernel taint
[   60.340140] INFO: Slab 0xffffea0001440600 objects=32767 used=65408 fp=0x          (null) flags=0x5fffff80000000
[   60.343095] CPU: 0 PID: 8634 Comm: sh Tainted: G    B           3.19.0-next-20150213-sasha-00045-g897c679-dirty #1920
[   60.345315]  ffff88001fc0ee40 00000000542c5de6 ffff88001114f908 ffffffffabb74948
[   60.346454]  0000000000000000 ffffea0001440600 ffff88001114f9e8 ffffffffa1764b4f
[   60.347582]  0000000000000007 ffff880000000028 ffff88001114f9f8 ffff88001114f9a8
[   60.349842] Call Trace:
[   60.350243]  [<ffffffffabb74948>] dump_stack+0x4f/0x7b
[   60.350975]  [<ffffffffa1764b4f>] slab_err+0xaf/0xd0
[   60.351714]  [<ffffffffa307cce0>] ? memchr_inv+0x2c0/0x360
[   60.352592]  [<ffffffffa17671b0>] slab_pad_check+0x120/0x1c0
[   60.353418]  [<ffffffffa17673a4>] __free_slab+0x154/0x1f0
[   60.354209]  [<ffffffffa141c365>] ? trace_hardirqs_on_caller+0x475/0x610
[   60.355168]  [<ffffffffa1767478>] discard_slab+0x38/0x60
[   60.355909]  [<ffffffffa176d478>] __kmem_cache_shrink+0x258/0x300
[   60.356801]  [<ffffffffa1764770>] ? print_tracking+0x70/0x70
[   60.357621]  [<ffffffffa1764770>] ? print_tracking+0x70/0x70
[   60.358448]  [<ffffffffa16c8460>] kmem_cache_shrink+0x20/0x30
[   60.359279]  [<ffffffffa17665db>] shrink_store+0x1b/0x30
[   60.360048]  [<ffffffffa17647af>] slab_attr_store+0x3f/0xf0
[   60.360951]  [<ffffffffa1764770>] ? print_tracking+0x70/0x70
[   60.361778]  [<ffffffffa193c56a>] sysfs_kf_write+0x11a/0x180
[   60.362601]  [<ffffffffa193c450>] ? sysfs_file_ops+0x170/0x170
[   60.363447]  [<ffffffffa1939ea1>] kernfs_fop_write+0x271/0x3b0
[   60.364348]  [<ffffffffa17ba386>] vfs_write+0x186/0x5d0
[   60.365112]  [<ffffffffa17bd146>] SyS_write+0x126/0x270
[   60.365837]  [<ffffffffa17bd020>] ? SyS_read+0x270/0x270
[   60.366608]  [<ffffffffa141c365>] ? trace_hardirqs_on_caller+0x475/0x610
[   60.367580]  [<ffffffffa3091d7b>] ? trace_hardirqs_on_thunk+0x3a/0x3f
[   60.368566]  [<ffffffffabbea3ad>] system_call_fastpath+0x16/0x1b
[   60.369435] Padding ffff880051018f50: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
[   60.370750] Padding ffff880051018f60: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
[   60.372077] Padding ffff880051018f70: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
[   60.373450] Padding ffff880051018f80: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5  kkkkkkkkkkkkkkk.
[   60.374763] Padding ffff880051018f90: bb bb bb bb bb bb bb bb c8 8d 01 51 00 88 ff ff  ...........Q....
[   60.376083] Padding ffff880051018fa0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
[   60.377457] Padding ffff880051018fb0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
[   60.378768] Padding ffff880051018fc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
[   60.380084] Padding ffff880051018fd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
[   60.381475] Padding ffff880051018fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
[   60.382798] Padding ffff880051018ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
[   60.384121] FIX kmalloc-64: Restoring 0xffff880051018f50-0xffff880051018fff=0x5a
[...]

And basically a lot more of the above.



Thanks,
Sasha

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